1. Technical Field
The present disclosure relates to an organic electroluminescent device (hereinafter referred to as an “organic EL device”), which is an electrical light-emitting device. In particular, the present disclosure relates to a low-power driving technique in a wide luminance range from low luminance to high luminance for light sources or the like.
2. Description of the Related Art
Organic EL devices are current-driven light-emitting devices and include a functional layer containing an organic material between a pair of electrodes, that is, an anode and a cathode. Organic EL devices are driven by utilizing electroluminescence. Electroluminescence is a luminous phenomenon generated by recombination of holes injected into a functional layer from an anode and electrons injected into the functional layer from a cathode in response to the application of a voltage between the anode and cathode. Organic EL devices advantageously have high visibility due to self-luminescence and high impact resistance because the organic EL devices are completely solid-state devices. Thus, organic EL devices receive attention as light-emitting devices or light sources in various display apparatuses.
In order to allow organic EL devices to efficiently emit bright light with low power consumption, it is important to efficiently inject carriers (holes and electrons) from electrodes into a functional layer. In order to efficiently inject carriers into a functional layer, an charge injection layer for lowering the injection energy barrier is generally disposed between electrodes and the functional layer. A hole-injection layer is disposed between a functional layer and an anode, and an electron-injection layer is disposed between a functional layer and a cathode.
Adv. Funct. Mater. 2008, 18, 145-150 discloses the use of titanium oxide in an electron-injection layer. Appl. Phys. Lett. 2007, 91, 223501 discloses the use of zinc oxide in an electron-injection layer. In this literature, a zinc oxide film is formed by spray pyrolysis. More specifically, the zinc oxide film is formed by spraying a cathode with a solution containing zinc acetate dihydrate and heat-treating the solution at 500° C. for 12 hours.